Detailed Description of the Invention
This invention relates to tarpaulins having greatly increased tearing strengths.
For sheets to cover goods placed outdoors, freight cars or vehicles, use has been made of canvas consisting of spun yarns such as cotton, hemp and poly(vinyl acetal) series fibers, one side or both sides of which are treated with paraffin or synthetic resins to provide a waterproof property. However, the canvas has such defects as heavy weight per unit area, comparatively great absorption of water, comparatively small strength per unit weight, poor durability and high cost of base fabrics resulting from the high cost of yarns which require a spinning process.
Recently, however, there has been demand for canvas of great strength, light weight and low cost, so that a filament of various synthetic fiber consisting of a single thread of a great strength has come into use. A filament of this type, since it consists of a continuous thread, has a high rate of utilization because of its strength, so that the canvas woven therefrom has many merits such as a great tensile strength, a small amount of thread, thereby leading to a decrease in the weight thereof, and the low cost of tarpaulins because of using the inexpensive threads requiring no spinning process. However, there are some defects in such tarpaulins made of base fabrics consisting of the synthetic fiber filaments. When the density of the base fabric is high or when the base fabric is treated with resin to fix the freedom of the fibers, the tensile strength thereof is great but the tearing strength is comparatively small. A tarpaulin used for trucks, for example, which is produced by weaving a base fabric from ordinary polyester filaments and waterproofing the base fabric, is defective in that when a small tear occurs in the tarpaulin, a large L-shaped laceration is caused and enlarges over a short period of time.
In order to overcome the above defects, a thick thread is woven into the base fabric to increase the tear strength thereof. In this case, it is considered that the thick thread may prevent the tearing of the base fabric, but in fact, the thick thread only indicates the presence of many fibers in part and so does little to prevent the strikingly accelerated tearing of the tarpaulin including the thick thread. Moreover, when a waterproof film is formed, the portion of the swollen thick thread causes the top portion of the film to break so as to lose the waterproofing effect. p One object of this invention is to provide tarpaulins, particularly having great tearing strengths, formed by adhering films of plastics, rubbers or mixtures thereof to one side or both sides of base fabrics woven from filaments or spun yarns by application or impregnation. The fundamental principle of this invention resides mainly in using base fabrics for tarpaulins woven from a mixture of a principal thread and another thread which is different from the principal thread in physical properties or in using base fabrics for tarpaulins provided by twisting a binding yarn which is different from the principal thread in physical properties around the principal thread arranged in warp and woof so as to increase the tear strength of a base fabric by mingling the principal thread with the thread different therefrom in physical properties in either case. This invention can be applied not only to woven fabrics of filaments but also to woven fabrics of spun yarns.